As the family slept, a silent killer slipped undetected into their Lubbock home  through the air vents.

The next morning, all four were taken to the emergency room at St. Mary's Hospital (now known as Covenant Medical Center-Lakeside) with deadly levels of carbon monoxide in their bloodstreams.

Carbon monoxide is the most common cause of accidental poisoning death in the United States. According to the Centers for Disease Control and Prevention, carbon monoxide poisoning accounts for more than 2,100 accidental deaths and more than 20,000 health-related injuries each year.

At high levels, carbon monoxide can kill a person in minutes. At moderate levels, it can cause severe headaches, dizziness, mental confusion, nausea or fainting. Mild levels can cause shortness of breath, mild nausea and headaches as well.

Denae McLellan woke up about 1 a.m. Oct. 26, 1997, because she thought she heard her 1-year-old daughter, Macy, having a bad dream.

''I felt nauseous, and my head was pounding,'' she recalled. ''I thought I was getting the flu or a stomach bug.''

She returned to her bed but later heard both children, the baby and son Landon, who was 3 years old at the time, waking up throughout the remainder of the night.

At 6 a.m., Denae awoke again. She walked from her bedroom to the living room, where she passed out, falling on her son's blocks. She regained consciousness and made her way back to her bedroom, where she fainted again.

Johnny McLellan, who also had a headache throughout the night, woke up to find his wife lying on the floor. He helped his wife back to bed, then made his way through the house.

''I checked the heater and everything was fine,'' Johnny recalled. ''But the day before, I had changed the furnace filter and left the blower door off. Strong winds that night also caused an updraft that circulated carbon monoxide back through the air vents.''

He was unaware that the nausea he and his wife were experiencing was one of the ill effects of those deadly fumes, suspecting food poisoning or a stomach virus instead.

He rejoined his wife in bed, but when the baby awoke again at 7:30 a.m. neither parent could muster the strength to get up.

''It wasn't that we couldn't move,'' Denae said. ''We just couldn't get out of bed.''

They called Johnny's sister, Judy, to come to the house to help take care of their kids, but she ended up driving the entire family to the emergency room, a decision that likely helped save their lives.

''We felt coherent, but we probably weren't at that point,'' Johnny remembered.

When questioned at the emergency room, Denae knew who was president of the United States but she could not remember the year. All four family members had blood tests, which detected the dangerous levels of carbon monoxide in the four McLellans. They were rushed over to the hyperbaric chambers at Covenant Health System's Total Wound Treatment Center.

With hyperbaric oxygen therapy, a patient is administered oxygen at greater than normal pressure. The patient is placed in a specially-designed chamber and receives 100-percent oxygen while the pressure of the chamber is increased slowly.

''It has been used in treating carbon monoxide for 25 years as a standard of care,'' said Dr. Randall Wolcott, medical director for the Total Wound Treatment Center. His father, Dr. Lester Wolcott, was instrumental in getting the hyperbaric chambers in Lubbock in 1996.

''When you breathe in carbon monoxide in high concentration, it binds with the hemoglobin and ties it up so it can't carry oxygen. It's a race against time to get the carbon monoxide out before cell death occurs. If we can get a patient into a hyperbaric chamber within the first six hours, you can save cells. If you get the patient into the chamber within two hours, they have a better outcome.''

Johnny and Landon spent 90 minutes together in the same hyperbaric chamber. Denae had been placed in the chamber with her baby but had to be removed after her face began swelling from the pressure. It was then determined that she had suffered a fractured cheek bone from her fall in the living room.

Denise Cox, the unit director at the Total Wound Treatment Center, volunteered to take the baby back into the chamber. When the baby's treatment was completed, Denae was reinserted into the hyperbaric chamber with the pressure decreased.

''If you can get high doses of oxygen into the cell before the cell dies, you can wash out the carbon monoxide molecule,'' Wolcott said. ''With an hour or two in the chamber, the carbon monoxide gets washed out.''

For two weeks following their ordeal, Johnny and Denae McLellan suffered such side effects as tunnel vision, shaking and short-term memory problems. The complications gradually cleared up, though the effects on memory lingered for a month or more.

''It would've been a lot easier had we had a carbon monoxide detector,'' Denae said. ''We don't have an old house. We don't have gas appliances. We didn't think we fit the criteria to get one.''

The McLellans have taken aggressive steps to alert their family members and friends of their experience, sending out a letter detailing an account of their incident and explaining the importance of getting a carbon monoxide detector.

''We've told practically everyone we know about the importance of having a carbon monoxide detector in your home,'' Johnny McLellan said. ''You think it's not going to happen to you. That was us too.

Charles L. Ehrenfeld can be contacted at 766-8796 or cehrenfeld@lubbockonline.com